Optical 2×2 switches are known which have two input ports and two output ports and which are configurable in either a cross or a bar configuration. When set in the bar configuration, a signal received at a first input port IP1 is connected to a first output port OP1 and a signal received at the second input port IP2 is connected to a second output port OP2. When in the cross configuration the signal at the first input port IP1 is instead connected to the second output port OP2 and likewise the signal at the second input port IP2 is connected to the first output port OP1. This functionality enables information arriving at an input to be steered to either of the outputs and is well known in communication networks for routing. Groups of 2×2 switch nodes can be combined to give switching between more than two inputs and outputs if required.
Such switched optical nodes have application in computers and in communications networks. For example, packetized information may be sent to an input which carries with it a label containing routing information in the form of a destination address AMN where M identifies the label containing the address and N is an integer value which indicates which one of N output ports of the switch the incoming signal is to be passed to. An address may indicate a final destination on a network, for example, with the switched node knowing which output port links to that address by being aware of its location within the network. Alternatively, it may indicate directly a specific output port of the switch. When a packetised signal arrives at an input node of the switch, this routing information is extracted and used to set the switch in either the cross or bar configuration so as to provide the correct route to the destination.
A problem arises if two signals arrive, at respective input ports, at the same time, and both want to be sent to the same output port. To ensure that the switch behaves predictably this can be overcome by associating a priority value with each input port, one being given a higher priority than the other, so that most times there will always be one incoming signal with a higher priority than the other. Any information arriving at the input with the high priority will get its requested routing and the information at the other port will be routed to its correct output port only if possible. If not, due to a conflict, a conflict will be signalled.
To date, such optical 2κ2 switches have been implemented using a combination of optical and electronic components. The priority information and address information has been extracted from the incoming signals by converting it to an electrical signal for processing. An electrical processing circuit has then been provided which takes these signals and from them provides the drive signals for setting the switch in the cross or bar configuration and indicating a conflict if appropriate. This electronic domain processing and the need to convert from the optical to the electronic domain slows the throughput of the switch and has been found to be limiting in the maximum rate at which signals can be passed.